The present invention relates to the field of plant molecular biology, more particularly to regulation of gene expression and enhancing disease resistance in plants.
Disease in plants is caused by biotic and abiotic causes. Biotic causes include fungi, viruses, bacteria, and nematodes. Of these, fungi are the most frequent causative agent of disease on plants. Abiotic causes of disease in plants include extremes of temperature, water, oxygen, soil pH, plus nutrient-element deficiencies and imbalances, excess heavy metals, and air pollution.
A host of cellular processes enables plants to defend themselves from disease caused by pathogenic agents. These processes apparently form an integrated set of resistance mechanisms that is activated by initial infection and then limits further spread of the invading pathogenic microorganism.
Subsequent to recognition of a potentially pathogenic microbe, plants can activate an array of biochemical responses. Generally, the plant responds by inducing several local responses in the cells immediately surrounding the infection site. The most common resistance response observed in both nonhost and race-specific interactions is termed the xe2x80x9chypersensitive responsexe2x80x9d (HR). In the hypersensitive response, cells contacted by the pathogen, and often neighboring cells, rapidly collapse and dry in a necrotic fleck. Other responses include the deposition of callose, the physical thickening of cell walls by lignification, and the synthesis of various antibiotic small molecules and proteins, among which are the pathogenesis-related (PR) proteins. Genetic factors in both the host and the pathogen determine the specificity of these local responses, which can be very effective in limiting the spread of infection.
Pathogenesis-related proteins, which have been described in a number of plants (see Bowles (1990) Ann. Rev. Biochem. 59:873-907 for review), include the PR-1 proteins. Although their biochemical functions remain unknown, expression of PR-1 proteins is generally induced by pathogens and many abiotic treatments associated with the elicitation of the defense response, more particularly a hypersensitive response (see WO 89/02437 for a review). In tobacco, PR-1 protein expression is induced by viral infection and salicylic acid (SA) treatment (van Loon et al. (1987) Plant Mol. Biol. 9:593; Ward et al. (1991) Plant Cell 3:1085). Barley plants resistant to powdery mildew caused by Erysiphe graminis accumulate PRb-1 (a basic PR-1) mRNA 12 hours after inoculation with that pathogen, while susceptible plants do not, indicating these proteins serve as antipathogenic agents that contribute to disease resistance. Ethylene, jasmonic acid (JA), and SA also induce the accumulation of PRb-1 in the resistant cultivars, but not in related susceptible lines (Muradov et al. (1993) Plant Mol. Biol. 23:439). Salicylic acid induces PR-1 protein accumulation in maize leaves. Ultraviolet light and C. carbonum (tox-) inoculations induce protein accumulation in Pr (hml) leaves (in Crane et al. (1996), Biology of Plant-Microbe Interactions (International Society for Molecular Plant-Microbe Interactions), pp. 223-226). These observations make maize PR-1 genes and their promoters ideal candidates for use in the development of transgenic plants, particularly transgenic plants having enhanced disease resistance.
Thus, isolation and characterization of PR-1 genes and their corresponding promoters, which can serve as regulatory regions for expression of their native gene or other heterologous nucleotide sequences of interest, are needed for genetic manipulation of plants to exhibit specific phenotypic traits, particularly enhanced disease resistance, either in response to a given stimulus or in a constitutive manner.
Compositions and methods for regulating gene expression in a plant are provided. Compositions are novel nucleotide sequences for inducible and constitutive plant promoters, more particularly promoters isolated from a family of maize genes encoding pathogenesis-related PR-1 proteins. Methods for regulating expression of a nucleotide sequence of interest in a plant using the promoter sequences disclosed herein are provided. The methods comprise transforming a plant cell with a nucleotide sequence of interest that is operably linked to one of the plant promoters of the present invention and regenerating a stably transformed plant from the transformed plant cell. Where the promoter is an inducible promoter of the invention, exposure of the transformed plant, or a particular tissue of the plant, to a stimulus activates, within the exposed tissues of the plant, expression of the nucleotide sequence operably linked to the particular inducible promoter disclosed herein.
Compositions of the invention also include the nucleotide sequences for novel maize PR-1 genes and the predicted amino acid sequences for the pathogenesis-related proteins encoded thereby. These nucleotide sequences are useful in the genetic manipulation of any plant when operably linked to a promoter that drives expression of a coding sequence in a plant cell, more particularly the PR-1 inducible and constitutive promoters disclosed herein. In this manner, transformed plants and progeny having increased resistance to pathogens and their related diseases may be obtained.